PADGEM (platelet activation dependent granule-external membrane protein) is an integral membrane protein of the alpha granules of platelets and Weibel-Palade bodies of endothelial cells that is expressed on the plasma membrane upon cell activation and granule secretion. Activated platelets, but not resting platelets, bind to neutrophils, monocytes, HL90 cells, and U937 cells. This interaction is inhibited by anti-PADGEM antibodies, PADGEM, and EDTA; anti-GPllb-Ma, anti-thrombospondin, anti-GPIV, and thtombospondin produce no effect. Neutrophils and U997 cells, in contrast to Jurkatt cells, contain PADGEM recognition sites, as shown by binding of PADGEM contained in phospholipid vesicles. These results indicate that PADGEM mediates adhesion of activated platelets to monocytes and neutrophlls. Therefore, PADGEM shares not only structural but also functional homology with ELAM-1 and MEL-14, members of a new family of vascular cell adhesion molecules.